Switched capacitor circuits are frequently used in low power applications, such as analog/digital (A/D) converters and filter applications, for example. These circuits operate in discrete time and typically include several stages. Each stage frequently includes an operational amplifier or an operational transconductance amplifier. These components consume energy during the whole duty cycle and the settling time for the output voltage can be quite long as the output signal settles exponentially with time.
A conventional switched capacitor sampling stage is shown in FIG. 8 and a switched capacitor gain stage is shown in FIG. 9. The offset on the input stage is shown in the graph of the input signal in FIG. 10. The output of the gain stage 500 may be expressed as Vin×Cin/Cfb. The settling time for this output signal is shown in the graph of FIG. 11. If fourteen bits of accuracy are required for the output, then ten time constants are required for the output to settle sufficiently. During the time that the output voltage is settling, the stage is using power to generate the output voltage. Increasing power can result in an increase in speed for generating the output for the circuit; however, this action causes the circuit to consume more power.
To address the issues arising from switched capacitor circuits, comparator based/zero crossing based switched capacitor circuits were designed and implemented. The sampling stage of these circuits is generally the same as the one shown for the switched capacitor circuit in FIG. 8. In the gain stages for these types of circuits, an example of which is shown in FIG. 12, the operational amplifier or operational transconductance amplifier is replaced with a comparator 604 and a controlled current source 608. In response to the comparator detecting a virtual ground condition at its inputs, the comparator switches off the controlled current source so the current source no longer drains current. Because the current source is turned on for only a relatively short period of time, these circuits reduce overall energy consumption. Additionally, the current source ramps the output voltage linearly, which yields a faster settling time and shorter duty cycle. Moreover, comparators are simpler than operational amplifiers or operational transconductance amplifiers, and therefore, can typically be implemented using less area.
The input signal and output signal for a comparator based/zero crossing based circuit are shown in FIG. 13 and FIG. 14, respectively. While the linear forms for these signals provide faster response times, the overshoot in these signals result in a positive offset. This overshoot produces both an offset error and a linearity error in the signals. These errors are influenced by certain parameters of the circuit, such as comparator delay and the non-linear response of some components in the circuit, such as switches. To address the offset error, some efforts have been made to use a second controlled current source with a much smaller value for fine adjustment.